[Metallocene Compounds]
In recent years, metallocene compounds are well known as homogeneous catalysts for olefin polymerization. After the report of isotactic polymerization by W. Kaminsky et al. (see Non Patent Literature 1), many studies have been made on olefin polymerization, in particular, stereoregular α-olefin polymerization using metallocene compounds.
In α-olefin polymerization using metallocene compounds, it is known that the stereoregularity and the molecular weights of the obtainable α-olefin polymers are greatly varied by the introduction of substituents into the cyclopentadienyl ring ligands of the metallocene compounds or by the bridging of the two cyclopentadienyl rings.
[Bridged Metallocene Compounds]
For example, there are the following reports as to propylene polymerization catalyzed by metallocene compounds which have a ligand in which a cyclopentadienyl ring and a fluorenyl ring is bridged to each other.
From the viewpoint of stereoregularity, dimethylmethylene(cyclopentadienyl)(fluorenyl)zirconium dichloride affords syndiotactic polypropylene (see Non Patent Literature 2); dimethylmethylene(3-methylcyclopentadienyl)(fluorenyl)zirconium dichloride having a methyl group at the 3-position of the cyclopentadienyl ring affords hemiisotactic polypropylene (see Patent Literature 1); and dimethylmethylene(3-tert-butylcyclopentadienyl)(fluorenyl) zirconium dichloride having a tert-butyl group at the 3-position of the cyclopentadienyl ring affords isotactic polypropylene (see Patent Literature 2).
Dimethylmethylene(3-tert-butyl-5-ethylcyclopentadienyl) (3,6-di-tert-butylfluorenyl)zirconium dichloride having tert-butyl groups at the 3- and 6-positions of the fluorenyl ring affords polypropylene with higher isotactic stereoregularity than obtained with dimethylmethylene(3-tert-butyl-5-methylcyclopentadienyl)(fluorenyl)zirconium dichloride (see Patent Literature 3).
From the viewpoint of molecular weights, diphenylmethylene(cyclopentadienyl)(fluorenyl)zirconium dichloride having a cyclopentadienyl ring and a fluorenyl ring bridged via diphenylmethylene affords syndiotactic polypropylene having a higher molecular weight than obtained with dimethylmethylene(cyclopentadienyl)(fluorenyl)zirconium dichloride (see Patent Literature 4); diphenylmethylene(3-(2-adamantyl)-cyclopentadienyl)(fluorenyl)zirconium dichloride having a diphenylmethylene bridge affords isotactic-hemiisotactic polypropylene having a higher molecular weight than obtained with dimethylmethylene(3-(2-adamantyl)-cyclopentadienyl)(fluorenyl)zirconium dichloride (see Non Patent Literature 3); and dimethylmethylene(3-tert-butyl-5-methylcyclopentadienyl)(fluorenyl)zirconium dichloride having a methyl group at the 5-position of the cyclopentadienyl ring (the α-position relative to the bridge) affords isotactic polypropylene having a higher molecular weight than obtained with dimethylmethylene(3-tert-butylcyclopentadienyl)(fluorenyl) zirconium dichloride (see Patent Literature 5).
Further, dimethylmethylene(3-tert-butyl-2-methylcyclopentadienyl)(fluorenyl)zirconium dichloride and diphenylmethylene (3,4-dimethylcyclopentadienyl)(fluorenyl) zirconium dichloride having substituents at two adjacent positions on the cyclopentadienyl ring afford polypropylene having a lower molecular weight than obtained with dimethylmethylene(3-tert-butyl-5-methylcyclopentadienyl)(fluorenyl)zirconium dichloride and diphenylmethylene(3-methylcyclopentadienyl)(fluorenyl)zirconium dichloride, respectively (see Patent Literatures 5 and 6).
[5-Membered Ring-Bridged Metallocene Compounds]
A study reports the polymerization of propylene catalyzed by a metallocene compound in which a cyclopentadienyl ring and a fluorenyl ring are bridged via a 5-membered ring. However, such metallocene compounds have low usefulness in industry because of the fact that the stereoregularity of the obtainable polypropylenes is very low (see Non Patent Literature 4).
A recent study reports a metallocene compound having a cyclopentadienyl ring and a fluorenyl ring bridged via a 5-membered ring which can afford polypropylene having relatively high stereoregularity (see Patent Literature 7).
These metallocene compounds mentioned above exhibit excellent polymerization performance. In some applications, however, the catalysts are often required to afford polymers having still higher stereoregularity or still higher molecular weight with higher economic efficiency, namely, with high catalytic activity even under high-temperature polymerization conditions. Improvements are thus required.
[Metallocene Compounds Having Substituted Indenyl Ligands]
According to reports, metallocene compounds having substituted indenyl ligands afford relatively high stereoregularity or molecular weight (see Patent Literatures 8 and 9). However, such compounds are unsatisfactory in terms of performance under economically efficient polymerization conditions.
Because metallocene compounds are soluble in reaction media, they are generally used to catalyze polymerization in the form of supported catalyst systems in slurry polymerization or gas phase polymerization. Specifically, the metallocene compounds are supported on solid carriers. However, it is known that the polymerization performances such as stereoregularity control of the aforementioned compounds are markedly decreased when they are used in the supported form on carriers as compared to in the absence of carriers.
[Metallocene Compounds Having Substituted Azulenyl Groups]
To solve such problems, for example, a recent study reports a metallocene compound having a substituted azulenyl group as a ligand (see Patent Literature 10). However, even such catalysts do not achieve sufficient performances such as stereoregularity control when the polymerization temperature is elevated to obtain economic efficiency or when the compounds are supported on solid carriers.
Under these circumstances, there have been demands for further improvements in the catalytic performances such as polymerization activity, stereoregularity control and molecular weight control of polymerization catalysts including metallocene compounds (hereinafter, also written as “metallocene catalysts”).
[Olefin Polymers]
Olefin polymers, in particular, crystalline isotactic polypropylenes are widely used in various forming fields because of their inexpensiveness and excellent mechanical properties and chemical resistance.
Isotactic polypropylenes are known to have various-phase structures such as α-structure, β-structure, γ-structure and mesophase structure (see Non Patent Literature 5). Of these, the α-phase is the most stable structure. On the other hand, the β-phase outperforms the α-phase in such properties as impact resistance (see Patent Literature 11). Methods have been disclosed in which the surface roughness of films is controlled or porous films are produced by utilizing the nature of the polypropylenes being transitioned from the β-phase to the α-phase upon treatment such as heating or stretching (Patent Literatures 12 and 13).
Because the β-phase is a metastable structure, this phase is scarcely formed in Ziegler-Natta-catalyzed polypropylene under usual crystallization conditions irrespective of the primary structure of the polymer (see Non Patent Literature 6), and is formed only under specific crystallization conditions. Specifically, the β-phase is obtained by a temperature gradient method or by adding a large amount of a β-phase nucleator (see Non Patent Literature 7). In, for example, Patent Literature 13, the occurrence of a β-phase is evaluated based on β-activity as the indicator using differential scanning calorimetry (DSC). However, the β-activity is exhibited as a result of the addition of a β-phase nucleator, and thus resins free from β-phase nucleators show no β-activity.
[Isotactic Polypropylenes]
As mentioned earlier, there have recently been active studies and developments of metallocene-catalyzed isotactic polypropylenes. Metallocene-catalyzed isotactic polypropylenes have a uniform primary structure and hence exhibit excellent properties. Thus, a success in allowing metallocene-catalyzed isotactic polypropylenes to be formed into a β-phase will highly increase the usefulness of the polymers.
Unlike Ziegler-Natta-catalyzed polypropylenes, metallocene-catalyzed isotactic polypropylenes contain regioerrors ascribed to 2,1-insertions and 1,3-insertions in the primary structure (see Non Patent Literatures 8 and 9). As a result, the metallocene-catalyzed polypropylenes are prone to take a γ-phase form as compared to the Ziegler-Natta-catalyzed polypropylenes (see Non Patent Literature 10), and there is a study reporting that a sufficient amount of β-phase is not formed even when a β-phase nucleator is added (see Non Patent Literature 11).
A further study reports a metallocene-catalyzed polypropylene which is obtained by polymerization at an extremely low temperature and which consequently has a meso pentad fraction of greater than 99.9% and a very small amount of regioerrors ascribed to 2,1-insertions and 1,3-insertions according to 13C-NMR (see Patent Literature 14). In Patent Literature 14, the melting point of the phases is measured by DSC, but there is no description as to whether any melting point assigned to a β-phase was observed.
A recent study reports a metallocene-catalyzed isotactic polypropylene having high stereoregularity and a relatively small amount of regioerrors ascribed to 2,1-insertions and 1,3-insertions (see Non Patent Literature 12). In Non Patent Literature 12, the melting point of the phases is measured by DSC, but there is no description as to whether any melting point assigned to a β-phase was observed.